There are already known positioning devices, in which a magnetic field, alternating in its flux density due to positional changes, generates an electrical signal proportional to the flux density in a sensor, permitting the localization of the part to be positioned. These positioning devices allow not only a linear, but also a circular localization. In particular, in the case of internal combustion engines it is common practice to introduce a diaphragm ring between the poles of a magnet and, thus, to change alternately the magnetic flux density, or to make an annular magnet, having an alternating reversed polarity at its periphery, interact with a Hall sensor to generate the signals for the firing point of each cylinder. In these known devices the external magnetic fields, the temperature variations, and the manufacturing and mounting errors have a diminishing effect on the accuracy. Besides, the diaphragm ring is to be made of a special material with high permeability. The magnetic field only varies from a maximum value to a rectified minimum value, which makes high demands on the strength of the magnet. In the signal generators being fitted with Hall sensors and annular magnets, the controls of the switching threshold of the Hall sensors, the spacing tolerances between the annular magnet and the Hall sensor, and the controls of the annular magnet due to its manufacture and the bearing defects of the camshaft can not be taken into account much enough or only by special precautions.